It is a mode of being that is free from mind-contaminants (Kilesa)
such as lust, anger or craving. It is thus a state of great inner peace
and contentment - the end of suffering, or Dukkha. The Buddha in the
Dhammapada
says of Nirvana that it is "the highest happiness." This is not the
transitory, sense-based happiness of everyday life, but rather an
enduring, transcendental happiness integral to the calmness attained
through enlightenment.

The Buddha describes the abiding in nirvana as 'deathlessness'
(Pali:
amata or amaravati) or 'the unconditioned' and as the highest spiritual
attainment, the natural result that accrues to one who lives a life of
virtuous conduct in accordance with Dharma. Such a life (called
Brahmacarya in India) dissolves the causes for future becoming (Skt,
Karma; Pali, Kamma) that otherwise keep beings forever wandering
through realms of desire and form (samsara).

This is mostly the Theravada interpretation of Nirvana. Mahayana
Buddhism complicates things a bit with its Bodhisattva
ideal. The Bodhisattva actually gives up full enlightenment in
order to 'help and save humanity'. Still, divine Bodhisattvas are
considered enlightened compared to 'normal human beings'. They are in
effect in the possession of the knowledge of the path leading to
enlightenment, because otherwise they would not have been able to give
up full Nirvana.

Moksha in Hinduism

On Moksha in Hinduism wikipedia mentions the following:

In Hinduism, liberation occurs when the individual soul (human
mind/spirit) or jīvatman recognizes its identity with the Ground of all
being - the Source of all phenomenal existence known as Brahman. It is
technically incorrect, nonetheless, to view them - both often spoken of
as Self - as a monist being of sorts, something possessing substances,
qualities or attributes. In actuality, Hindu scripture like the
Upanishads and Bhagavad Gita, and especially the works of the
non-dualistic Hindu school, Advaita Vedanta,
say that the Self or Super-Soul is formless, beyond being and
non-being, beyond any sense of tangibility and comprehension. Moksha is
seen as a final release from one's worldly conception of self, the
loosening of the shackle of experiential duality and a re-establishment
in one's own fundamental nature, though the nature is seen as ineffable
and beyond sensation. The actual state of salvation is seen differently
depending on one's beliefs.

In Advaita
philosophy, the ultimate truth is not a singular Godhead, per se, but
rather is oneness without form or being, something that essentially is
without manifestation. Moksha is union with this oneness. The concepts
of Moksha and Buddhist Nirvana are comparable. Indeed, there is much
overlap in their views of consciousness and attainment of
enlightenment. For liberal Advaitists, Moksha is seen as complementing,
rather than denying, the 'voidness' of Buddhism.

In dualist and qualified advaitic Hinduism, Moksha means union
or close association with God. See Krishnology.

The debate between Hinduism and Buddhism on enlightenment is an
eternal one. It started when the Buddha denied that there was anything
permanent in consciousness (the fifth skandha). It hasn't ended since.
There are people who see mostly the convergence of experience and
doctrines in (especially) Advaita Vedanta and Mahayana Buddhist
'Emptiness'. One example of this view is an article by Bijoy H. Boruah 'Atman in
Sunyata and the Sunyata of Atman'.